Could I have SIM Locked my phone?

Hi, I live in the UK. I purchased a SIM Free HTC ONe M8 from The Carphone Warehouse and converted it to a GPE. I wanted to check to see if the handset was unlocked to all networks but I didn't have another networks SIM card so I went into the fastboot menu and clicked on SIM lock but nothing happened. How can I check without using another SIM card? I'm worried that I've locked the phone by clicking SIM lock in the fastboot menu. Thanks